1. Field of the Invention
The present invention relates to an optical communication system, an optical communication unit, and an optical transceiving package that form an optical access network, and particularly, a wavelength-division multiplexing access network.
2. Description of the Related Art
FIG. 16 shows a structural example of an Ethernet-based access system. In FIG. 16, a plurality of subscriber units 1051 and an Ethernet switch/router 1053 located in a center unit 1052 are connected together respectively via one or two optical fibers 1031 so as to enable bi-directional transmissions to be performed. Here, a Fast Ethernet having a physical bit rate of 125 Mbps and a maximum throughput of 100 Mbps, and a Gigabit Ethernet having a physical bit rate of 1.25 Gbps and a maximum throughput of 1 Gbps or the like are assumed. Note that the term physical bit rate refers to the rate at which electrical signals or optical signals are physically switched on and off, regardless of the actual throughput.
FIG. 17 shows a structural example of a wavelength-division multiplexing access network. In FIG. 17, a plurality of subscriber units 1051 and a center unit 1052 are connected via optical fibers 1031, an optical splitter unit 1056, and an optical fiber 1032. An arrayed waveguide grating (AWG) filter 1057 of the optical splitter unit 1056 multiplexes upstream optical signals from each of the subscriber units 1051 and transmits them to the center unit 1052. In addition, the AWG filter 1057 demultiplexes downstream wavelength-division multiplexed optical signals transmitted from the center unit 1052 into each wavelength and transmits them to each subscriber unit 1051. The arrayed waveguide grating (AWG) filter 1054 of the center unit 1052 demultiplexes the upstream wavelength-division multiplexed optical signals multiplexed by the optical splitter unit 1056 into each wavelength, and transmits them to an Ethernet switch/router 1053. In addition, the AWG filter 1054 wavelength-division multiplexes downstream optical signals from the Ethernet switch/router 1053 that are addressed to each subscriber unit 1051, and transmits them to the optical splitter unit 1056.
Note that the structure shown in FIG. 17 is just an example, and it is also possible to employ a structure in which two optical fibers are used in order to separate the upstream optical transmission path from the downstream optical transmission path, and the upstream signals and the downstream signals are multiplexed or demultiplexed by separate arrayed waveguide grating filters.
It should be noted that research is proceeding on wavelength-division multiplexing access networks on the assumption that each subscriber unit transmits an optical signal of a different wavelength from the other subscriber units. However, recently, a scheme (see Document 1 (K. Akimoto, et al., “Spectrum-sliced 25-GHz spaced, 155 Mbps×32 channel WDM access”, Proc. CLEO/Pacific Rim 2001, ThB1-5, pp. II-556-II-557, Chiba (Japan), July, 2001)) is being investigated in which each subscriber unit transmits modulated broadband light, and its spectrum is sliced by an arrayed waveguide grating (AWG) filter of the optical splitter unit into different wavelengths allocated to each subscriber unit. The spectrum-sliced light of each wavelength is then wavelength-division multiplexed and transmitted to the center unit. As a result of this scheme, it is possible for the wavelength-division multiplexing access network to employ each subscriber unit using an optical transmitter having the same specifications. This allows the optical transmitter production costs and the costs of wavelength control to be reduced.
FIG. 18 shows a structural example of a wavelength-division multiplexing access network based on spectrum-slicing techniques. The basic structure is the same as that shown in FIG. 17; however, here, the upstream optical transmission is shown.
A broadband light source (not shown) that generates broadband spontaneous emissions (ASE) is provided in each subscriber unit 1051. Note that a subscriber unit may also be called an optical network unit (ONU). Examples of the broadband light source include light emitting diodes (LED), super-luminescent diodes (SLD), semiconductor optical amplifiers (SOA), and optical fiber amplifiers. LED, SLD, and SOA are semiconductor devices that are able to perform direct modulation. Therefore, when used as a transmitter, they are able to output the modulated ASE without external modulators. However, in the case of an optical fiber amplifier, it is necessary to modulate a spontaneous emission using an external modulator. Such modulated spontaneous emissions are known as “modulated broadband light”. The modulated broadband light output by each subscriber unit 1051 is shown by (a) in FIG. 19.
The arrayed waveguide grating filter 1057 of the optical splitter unit 1056 receives the modulated broadband light transmitted from each subscriber unit 1051 via the respective optical fiber 1031. Optical signals obtained by spectrum-slicing (spectrum-sliced light) are wavelength-division multiplexed, and transmitted to the center unit 1052 via the optical fiber 1032. The wavelength-division multiplexed spectrum-sliced light when there are 64 subscriber units 1051 is shown in (b) in FIG. 19. The arrayed waveguide grating filter 1054 of the center unit 1052 demultiplexes wavelength-division multiplexed optical signals transmitted via the optical fiber 1032 for each wavelength allocated to each subscriber unit.
However, as is also described in the Document 1, the transmittable signal rate when a spectrum-slicing scheme is used is limited by the filter characteristics (transmission spectral width) of the arrayed waveguide grating filter 1057 of the optical splitter unit 1056. This is because spontaneous emission is used instead of laser light for the optical carriers.
When spontaneous emission is transmitted, the beat noise generated by the interference between each spectral component of the spontaneous emission causes the signal-to-noise ratio to be degraded. FIGS. 20A to 20D show the results of a numerical calculation for explaining the effect of beat noise when using spectrum-slicing. FIG. 20A shows the computer-simulated optical spectrum of the white light that has a flat optical spectrum. FIG. 20B shows a temporal waveform after the spontaneous emission has been received by a 200 GHz bandwidth optical receiver. FIG. 20C shows an optical spectrum when a spontaneous emission having a flat spectrum is sliced using a 25 GHz full width at a half maximum arrayed waveguide grating filter. FIG. 20D shows a temporal waveform after the spectrum-sliced light has been received by a 200 GHz bandwidth optical receiver.
The beat noise generated by the white light (FIG. 20A) has such frequency characteristics that the spectrum is widely distributed from the low frequencies to approximately the same high frequencies as the optical spectral width (bandwidth). When this beat noise is received by the optical receiver that has an extremely narrow bandwidth relative to the optical bandwidth, most of the beat noise components are removed by this optical receiver, resulting in a relatively low noise temporal waveform, such as is shown in FIG. 20B, being obtained.
In contrast, the beat noise generated by the spectrum-sliced light (FIG. 20C) has such frequency characteristics that the spectrum is distributed from the low frequencies to approximately the same frequency as the bandwidth of the arrayed waveguide grating filter. When the bandwidth of the arrayed waveguide grating filter is approximately the same or less than as the bandwidth of the optical receiver, most of the beat noise components are not removed by the optical receiver, resulting in temporal waveforms having high intensity noise, as is shown in FIG. 20D.
In order to increase the number of subscriber units that can be accommodated in a wavelength-division multiplexing access network, it is necessary to narrow the wavelength spacing (this is known as the channel spacing) between wavelength-division multiplexed optical signals (or spectrum-sliced light). Therefore, it has been necessary to narrow the spectral width occupied by each channel, resulting in the transmittable rate for each subscriber being reduced.
A quantitative analysis of the above characteristics is found in Document 2 (J. S. Lee et al., “Spectrum-sliced fiber amplifier light source for multichannel WDM applications”, IEEE Photonics Technologies Letters, Vol. 5, pp. 1458–1461, 1993). This document describes how the signal-to-noise ratio can be determined as being approximately Bo/Be using the full width at a half maximum Bo of the arrayed waveguide grating filter and the electrical bandwidth Be of the optical receiver. A signal-to-noise ratio of approximately 144 or more corresponds to a bit-error rate of 10−9 or less, which is the transmission quality standard. For example, the rate that can be transmitted by spectrum-sliced light is restricted to approximately 170 Mbps in a 25-GHz spaced wavelength-division multiplexing system, on the assumption that the bandwidth of the optical receiver needs to be approximately 0.7 times the desired transmission rate, and the full width at a half maximum of the arrayed waveguide grating filter is approximately 0.7 times the channel spacing. Furthermore, when optical leakage from the other channels in the arrayed waveguide grating filter is considered, the transmittable rate using spectrum-slicing in a 25-GHz spaced wavelength-division multiplexing system is approximately 125 to 155 Mbps, which is described in the Document 1.
Here, if an attempt is made to transmit a 1.25 Gbps signal in a spectrum-sliced, 25-GHz spaced wavelength-division multiplexing system, the obtained signal-to-noise ratio is approximately 16 as the full width at half a maximum of the arrayed waveguide grating filter 1057 is approximately 0.7 times the channel spacing. This corresponds to a bit-error rate of above 0.01 so that, for example, there is practically 100% loss for a packet length of 16 bytes or more.
On the other hand, for a downstream optical transmission, in the center unit 1052, a reduction in cost is made possible using simultaneous wavelength control of multiple wavelengths (multiple channels), and multi-wavelength light sources (Japanese Unexamined Patent Application, First Publication No. 2002-82323; Document 3 (M. Fujiwara et al., “Flattened optical multicarrier generation of 12.5 GHz spaced 256 channels based on sinusoidal amplitude and phase hybrid modulation”, Electronics Letters, Vol. 37, No. 15, pp. 967–968, July, 2001)) and the like. Therefore, there is not the same need for the application of spectrum-slicing for upstream optical transmission. Here, the multi-wavelength light sources of Japanese Unexamined Patent Application, First Publication No. 2002-82323 and Document 3 are structured such that a dual multiplexing/modulating processing is performed by dividing into two the input light from 2n number of light sources that each generate light having a different single center wavelength, performing polarization combining on modulation results, and separating these into a plurality of carriers having different wavelengths so as to obtain the final output. Moreover, the multi-wavelength light sources of Japanese Unexamined Patent Application, First Publication No. 2002-82323 and Document 3 are structured such that phase modulation and amplitude modulation are performed on light having a single center wavelength using an electronic signal (for example, a sinusoidal wave) having a specific repetition rate, and multi-wavelength light having a plurality of center wavelengths is generated simultaneously by generating side bands.
With regard to the transmission rate, it is possible to transmit at least signals of 1.25 Gbps rate per channel (wavelength) using the multi-wavelength light sources of Japanese Unexamined Patent Application, First Publication No. 2002-82323 and Document 3, and this is made known in Document 4 (N. Takachio et al., “Wide area gigabit access network based on 12.5 GHz spaced 256 channel super-dense WDM technologies”, Electronics Letters, Vol. 37, pp. 309–310, March, 2001).
Accordingly, for example, as a scheme constructing a low-cost 25-GHz spaced wavelength-division multiplexing access network, if a spectrum-slicing scheme is used for the upstream signal transmission and a multi-wavelength light source is used for the downstream signal generation, then while the upstream transmission rate is restricted to approximately 155 Mbps or less, it is possible to provide a downstream transmission rate in the gigabit class. This enables a system to be achieved that is suitable for the downloading of a variety of contents files and the like.
In addition, as a different approach in a scheme for reducing the cost of upstream optical transmission, a carrier supply type of wavelength-division multiplexing access network is proposed in which an optical carrier for an upstream optical signal is supplied to each of a subscriber unit from the center unit, and a transmission is performed by modulating the optical carrier supplied to the subscriber unit (Japanese Unexamined Patent Application, First Publication No. 2000-196536, Document 5 (Takuya Nakamura et al., “A Study of Transmission Characteristics of Reflected-type Wavelength Division Multiplexing-Passive Optical Network System in Optical Signal Levels”, Technical Report of IEICE, OCS2000-50, pp. 13–18, September, 2000)). FIG. 21 shows an example of the structure of a carrier supply type of wavelength-division multiplexing access network.
In FIG. 21, a center unit 1152 is formed by a transmitting section 1160 and a receiving section 1161. The transmitting section 1160 is formed by a multi-wavelength generation/modulation section 1162 that generates downstream optical signals, a multi-wavelength generation section 1163 that generates optical carriers for the upstream optical signals, and a wavelength-division multiplexing (WDM) filter 1164 that multiplexes multi-wavelength light output from the multi-wavelength generation/modulation section 1162 and multi-wavelength generation section 1163.
Downstream optical signals from the center unit 1152 are sent to an optical splitter unit 1156 to which the center unit 1152 is connected via an optical fiber 1132. The downstream optical signals are demultiplexed for each wavelength by an AWG 1157 inside the optical splitter unit 1156, and are sent to the respective subscriber units 1151 via optical fibers 1131. They are then received by a receiver 1170 inside each subscriber unit 1151.
The optical carriers for upstream optical signals generated by the multi-wavelength generation section 1163 are sent to an AWG 1157 of the optical splitter unit 1156 along the same path as the downstream optical signals sent from the center unit 1152 to the subscriber units 1151. The AWG 1157 demultiplexes both the downstream optical signals and the optical carriers for each wavelength and sends them to the subscriber units 1151. In the subscriber units 1151, the downstream optical signals and the optical carriers are separated by a WDM filter 1171 and the optical carriers are input into optical modulators 1172. The optical modulators 1172 modulate the optical carriers to generate upstream optical signals. These are then sent to an AWG 1157 to which the optical modulators 1172 are connected via optical fibers 1141. The AWG 1157 performs wavelength-division multiplexing on the upstream optical signals from each subscriber unit 1151 and sends them to the center unit 1152 via the optical fiber 1142. The upstream optical signals are received by the receiving section 1161 of the center unit 1152.
Here, the wavelength arrangement is such that the downstream optical signals λ1, λ2, . . . , λN (downstream modulated light) and the optical carrier signals for the upstream optical signals λ1′, λ2′, . . . , λN′ (non-modulated light, upstream modulated light) (wherein N is the number of subscriber units 1151) use different wavelength bands.
The advantage of a carrier supply type of wavelength-division multiplexing access network is that it is not necessary to have a laser light source inside the subscriber unit, so that, because wavelength control is not necessary in the subscriber unit, the structure of a transmitter in the subscriber unit is simplified and a lowering in the cost of the wavelength-division multiplexing access network can be expected.
In the Fast Ethernet and Gigabit Ethernet, which are currently the most widely used bi-directional optical networks, the upstream and downstream physical bit rates are the same.
Accordingly, in a wavelength-division multiplexing access network in which a low cost spectrum-slicing scheme is used for the upstream signal transmissions with the transmission rate being restricted to approximately 155 Mbps or less, and that is able to provide a transmission rate in the Gigabit class for the downstream, which should be sufficient for the requirements of the subscriber, it is not possible for the aforementioned Fast Ethernet and Gigabit Ethernet signals to be transmitted in their existing state. This is because if Fast Ethernet is used it is not possible to make the downstream rate reach the Gigabit class, and if Gigabit Ethernet is used transmission of spectrum-sliced light is completely impossible due to the high rate of the upstream signals and the wide bandwidth.
If, however, a multi-wavelength light source is used as the light source for the upstream optical signals and downstream optical signals, if there is a fault in the light source unit there is a possibility that the optical carriers of all wavelengths will cease to operate. If this happens, a problem will arise in which all of the communications of all the subscriber units connected to the wavelength-division multiplexing access network will stop, thereby causing enormous damage.